Add support for multiple platform options in image load and save

[ctalledo]

- What I did

Prior to this change, docker image load and docker image save accept only a single platform via the --platform option.

This change adds support for multiple platforms using a comma-separated list passed to --platform. E.g.:

docker image save --platform linux/amd64,linux/arm64/v8 -o b.tar busybox:latest
docker image load --platform linux/amd64,linux/arm64/v8 -i b.tar

** NOTE **: Depends on the corresponding change in the Moby engine (see moby/moby#50166).

- How I did it

Updated the --platform option in the image load and image save commands to accept an array of platforms, as opposed to a single platform.

- How to verify it

Run updated unit tests.

- Human readable description for the release notes

`docker image load` and `docker image save` now supports multiple platform selection via `--platform` flag (e.g., `docker image load --platform linux/amd64,linux/arm64 -i image.tar`).

- A picture of a cute animal (not mandatory but encouraged)

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 55.05%. Comparing base (8b8f558) to head (0ba4362).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #6126      +/-   ##
==========================================
+ Coverage   55.02%   55.05%   +0.03%     
==========================================
  Files         361      361              
  Lines       30152    30161       +9     
==========================================
+ Hits        16591    16605      +14     
+ Misses      12604    12598       -6     
- Partials      957      958       +1     

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• ❄️ Test Analytics: Detect flaky tests, report on failures, and find test suite problems.

[vvoland]

Hmm I'm not sure I understand this one. How does terminal affect parsing here?

[ctalledo]

I was simply copying a comment in the test right above this line (i.e., "with-single-platform"), but the underlying rationale for the comment comes from this code:

func runLoad(ctx context.Context, dockerCli command.Cli, opts loadOptions) error {
...
	var options []client.ImageLoadOption
	if opts.quiet || !dockerCli.Out().IsTerminal() {
		options = append(options, client.ImageLoadWithQuiet(true))
	}
...

[vvoland]

Ah right.. We should apply these opts to a temporary var and then check the resulting opts struct, but we can't do that because these types are unexported..

I guess the only way to test it in this situation is by checking HTTP request at the HTTP client, but that would need rewriting the test a bit.

Although, I think the best solution would be to have a debug helper on the client side that would allow introspecting for testing purposes...

[vvoland]

We should do that in a follow up though

[thaJeztah]

FWIW; I had a "wip" branch to implement a multi-platform option, but it still needs some work (and a decision where we want to put it);

• opts: implement PlatformSlice option #6166

[vvoland]

Changed the changelog a bit, let me know if that looks good.

cc @ArthurFlag

[ctalledo]

Changed the changelog a bit, let me know if that looks good.

Thanks @vvoland, LGTM.